Fabrication of hollow articles



United States Patent 3,012,309 FABRICATION OF HOLLOW ARTICLES Milton F. Meissner, New Canaan, Conn, assignor to Olin Mathieson Chemical Corporation, East Alton, 131., a corporation of Virginia Filed May 18, 1956, Ser. No. 585,758 13 Claims. (Cl. 29-1573) This invention relates to the fabrication of hollow articles and more particularly to a method of expanding an unjoined interior portion of a blank to form a hollow article.

In a well-known process of fabricating hollow panels such as refrigerator heat exchangers and the like, a pattern of weld-inhibiting material is applied to a clean surface of a sheet of metal. A clean surface of a second sheet of metal is superimposed on this surface and the two sheets are secured to prevent relative movement and are welded together by hot rolling in the adjacent areas thereof which are not separated by the weld-inhibiting material. Hot rolling of the sheets results in reducing the thickness of the two sheets and elongating the resultant blank in the direction of rolling while the width of the resultant blank remains substantially the same as the initial width of the sheets. Following the hot rolling operation, the resultant blank is usually softened as by annealing to make it more pliable and if desired it may then be cold rolled and again softened as by annealing. The weldinhibiting material results in an unjoined portion between the outer surfaces of the blank. After softening the blank, the unjoined portion is expanded by injecting therein a fluid pressure of suflicient magnitude to permanently distend the blank in the area of the unjoined portion. The aforementioned process is fully disclosed in a patent to Grenell, U.S. 2,690,002, granted September 28, 1954.

Such blanks may be expanded after being clamped between dies having cut-out portions conforming to the configuration of the unjoined portion of the blank, or between spaced apart flat platens. If a blank is expanded between dies, the resultant hollow distended portion may be given any desired configuration. On the other hand, if the blank is expanded between spaced apart fiat platens, the configuration of the hollow distended portion cannot be controlled except to the extent of providing the outer portions of the distended hollow area with flat faces through their engagement with the fiat platens. By this mode of inflation the resultant article is necessarily limited to hollow areas of the same height. A blank may also be expanded without restraint of any sort but this is often unsatisfactory since the distention of the unjoined portion of the blank results in warping the blank.

It is often desirable that one portion of such a hollow article have a substantially greater volume than another portion as may be evidenced by an accumulator and tubes in a refrigerator heat exchanger. The most practical method of obtaining an increased volume in a portion of such a structure is to provide a greater unjoined width in the blank and then to outwardly distend the blank to a greater height in this area. Depending on the particular design of the unjoined portion of a blank, the pressure required to distend the narrow unjoined portions is often of such magnitude as to rupture the Wide unjoined portions. It quite often happens that to properly distend a relatively narrow unjoined portion of a blank will require an expansion pressure in excess of the rupture pressure of a relatively wide unjoined area so that some sort of external restraint is necessary to obtain proper expansion of a blank having both wide and narrow unjoined areas. Expansion between hard fiat platens will permit outward distention of the blank to only one height so that the wider unjoined portion cannot by this method be distended to a greater height than the narrow unjoined portions. A greater height in a portion of the panel may be obtained by expanding the unjoined portions of the blank without external restraint of any sort against the blank or distended portions thereof. However, at high pressures the resultant article will be warped and the edgesand distended portions of the article will tend to pull-in and curve because of the drawing eifect of the distended portions on the solid or web portion of the blank.

It is, therefore, an object of this invention to prevent warping of a blank during the expansion of an unjoined interior portion contained entirely within the outer surfaces of the blank to form a hollow article.

Another object of this invention is to prevent rupture of such a blank while the unjoined interior portion is being expanded to form a hollow article.

Another object of this invention is to expand the unjoined portion of such a blank to different heights in a single operation not requiring special dies and to retard warping of the blank.

Additional objects and advantages will be apparent from the following description and drawings in which:

FIGURE 1 is a perspective view of a sheet of metal having applied to a surface thereof a design of weldinhibiting material;

FIGURE 2 is a perspective view of the metal sheet of FIGURE 1 with a second metal sheet superimposed on the surface to which the weld-inhibiting material has been applied and showing the two sheets of metal being welded together in their adjacent areas not separated by the weldinhibiting material, by being passed through a pair of mill rolls, to form an integral blank;

FIGURE 3 illustrates an embodiment of this invention showing a transverse sectional view of the resultant blank of FIGURE 2 positioned between a pair of spaced apart compressible pressure pads and pad backing plates, the sectional view of the pattern of the unjoined portion of the blank being indicated generally by line 3-3 on the sheet in FIGURE 1;

FIGURE 4 is a sectional view taken on the line 44 in FIGURE 5 after the unjoined portion of the blank shown in FIGURE 3 has been expanded to form a hollow article;

FIGURE 5 is a perspective view of the expanded hollow article;

FIGURE 6 is a sectional view of another embodiment of pressure pads and baclc'ng plates after the pads have been clamped against a blank and the unjoined portion of the blank has been expanded to form a hollow article;

FIGURE? is a perspectiveview of another embodiment of a hollow article;

FIGURE 8 is a sectional view showing the blank from which the hollow article of FIGURE 7 is fabricated and still another embodiment of a pair of compressible pressure pads, the section of the blank being indicated generally by the line 9-9 in FIGURE 7;

FIGURE 9 is a sectional view similar to FIGURE 8 after the compressible pressure pads have been clamped against the blank and the unjoined portion of the blank expanded to form a hollow article, the section of the blank being taken on the line 99 in FIGURE 7;

FIGURE 10 is a perspective view of a modified form of a blank;

FIGURE 11 is a sectional view showing the blank of FIGURE 10 positioned between a pair of compressible pressure pads, the section of the blank being taken on line 1111 in FIGURE 10;

FIGURE 12 is a sectional view similar to FIGURE 11 after the compressible pressure pads have been clamped but showing another embodiment of a pair of spaced apart compressible pressure pads; and

FIGURE 14 is a sectional view similar to FIGURE 13 after the compressible pressure pads have been clamped against the blankand the unjoined portion of the blank expanded to form a hollow article.

Referring to the drawings, FIGURE 1 shows a sheet of metal 1, such as aluminum or copper, having applied to a clean surface 3 thereof a design of weld-inhibiting material 5. The serpentine portion 7 of'the design will ultimately form the tubes 9 in the hollow heat exchanger 11 shown in FIGURE 5. The larger area 13 of weld-inhibiting material 5 will ultimately form the large distention, such as an accumulator 15 in the heat exchanger 11 shown in FIGURE 5, The ends 17 and 19 of serpentine portion 7 and one end of the larger area 13 of weld-inhibiting material are interconnected by strips 21 of weld-inhibiting material;

One method of expanding the unjoined portion of the resultant blank is to inject afluid pressure therein. One

method of accomplishing this is to provide an opening into the unjoined portion by interconnecting the aforementioned pattern of weld-inhibiting material and an edge of the sheet with a strip 23 of weld-inhibiting material. Other methods of injecting a fluid pressure into the unjoined portion of a blank are known in the art, such as drilling into the unjoined portion and applying the expansion pressure through the resultant openings, or by applying a weld-inhibiting material which will expand after the sheets are welded together. After the blank has been welded together, the strip 23 of weld-inhibiting material provides a nozzle receiving strip 25 as shown in FIGURE 5.

A surface of a second sheet of metal 27 is superimposed on the surface 3 of the first sheet of metal to which the weld-inhibiting material 5 has been applied. The two' in the direction or" rolling. If desired, the resultant integral blank 33 having an unjoined interior portion may be softened in any appropriate manner asjbyra'nnealing and thereafter the blank may becold rolled to provide a more even thickness, and again annealed. This process of fabricating a blank having an unjoined interior portion is fully described in the aforementioned patent to Grenell. The invention, however, is not limited to blanks fabricated by such a process and is equally applicable to blanks fabricated in-many other manners such as by adhesively securing together two sheets of material such as plastic, polyethylene or metal with the adhesive omitted from a central portion of the sheets to define an unjoined interior portion. 7

FIGURE 3 shows the resultant blank 33 received between a pair of compressible pressure pads 35, in this instance compressible platens. The pressure pads may be either resilient, as a rubber-like pad, or non-resilient, as a balsa wood pad. The required compressibility of the pads will depend on the design of the unjoined portion 5 of the blank, the material from which the blank is fabricated, the thickness of the walls defining each side of 4 her. The shims control the initial compression of the compressible pressure pads 35 and the height of the shims may be varied depending. on the thickness and compressithe unjoined portion of the blank and in part on the desired outward distention or height of the distention required in the hollow article.

' toward each other until the upper backing member engages shims 39 placed on top of the lower backing membility of the pads and the force required to be applied against the blank 33. The backing members must be held together with such force that they will not be forced apart upon subsequent expansion of the unjoined portion of the blank. The backingv members may be attached to'the adjacent working faces of a conventional press, such as the press shown in the patent to Harrison, U.S. 1,329,969, granted February 3, 1920.

Prior to clamping the blank 33 between the pressure pads 35, the nozzle receiving strip 25 is forcedopen and a nozzle 41 is inserted into the resulting orifice. The backing plates 37, are then brought together until the upper plate engages the'shims 39, clamping the blank 33 between the compressible pressure pads 35.

In FIGURE 4, the blank 33 has been clamped between the compressible pressure pads 35 and the unjoined portion 5 of the blank has been-expanded by injecting therein, through the nozzle 41, afluid pressure of suflicient magnitude to permanently distend the blank in the area of the unjoined portion. During the first 17 to 30 percent compression, a rubber pad reacts much in the manner of a fluid and tends to flow around the distentions in the.

blank upon expansion of the unjoined portion of the blank. After 17 to 30 percent compression of the rubber has been exceeded, depending on the rubber hardness, the rubber reacts increasingly as a spring and as a rigid pad. Therefore, during the initial expansion of a blank, the

distention has much the same shapeas if expanded with out restraints of any sort, but the tendency of the blank to warp or rupture is retarded. After the initial range of compression of the rubber is exceeded, the distentions will tend to flatten on their outer surfaces as if the blank had been expanded between spaced apart rigid flat platens and upon release of the pressure with unjoined portion 5, the hollow portion may be collapsed by the spring effect ofthe rubber.

When the blank 33 has been distended between resilient compressible pressure pads 35 to form a hollow article 11, it is usually necessary that the pressure exerted within the hollow portion 9 and 15 of the article and the pressure exerted by the resilient pressure pads 35 on the outer surfaces of the hollow portion of the article be released in such a manner that the hollow portion will not be ruptured by the pressure therein nor that the force of the pads will collapse the hollow portion of the article. In other words, in most instances and especially when the hollow article includes a relatively large hollow portion, such as accumulator 15, releasing a the pressure exerted by resilient pads 35 .on the outer surfaces of the hollow portion will cause the hollow portion to burst since the pressure exerted within'thehollow portion is usually above the rupture pressure which the hollow article can withstand when externally unrestrained. Siinilarly,,if the pressure within the hollow portion of the article is released while the pressure of the resilient pads is maintained on the outside of the hollow article, the force of the resilient pads will collapse the hollow portion of the article. Therefore, it is usually necessary that the pressure within the article and the pressure of the pads be released concurrently so that the diiference in these pressures will never be such that the ruptured pressure of the hollow portion of the article is'exceeded nor-that the pressure which the pads exerted on the outer surface of the article exceeds the pressure within the hollow portion of the, article sufficiently to cause the hollow portion to collapse.

FIGURE 5 shows the hollow article 1 1 with the relatively wide hollow portion 15 distended to av greater 1 height than the adjacent relatively narrowserpentine tubes 9. By clamping the blank between compressible pressure pads 35, as shown'in FIGURE 3, any tendency of theblank 33 to warp upon, distention of the unjoined portion thereof is retarded by the clamping force of the pads against the solid or web portion blank. Similarly, the friction between the pressure pads and the blank retard the tendency of the blank to pull-in upon distention of the unjoined portion. Such pulling in of the blank causes the edges of the blank to be concavely curved toward the center of the blank. By trimming the edges of the blank after distention thereof, this effect may be eliminated so far as the periphery of the blank is concerned, the primary objection to such trimming being the added expense of the material so trimmed and the additional time and equipment required for such trimming operation. A more serious effect of such pulling in is that the hollow portions of the'finished article will also be deformed such as the tubes being curved toward the large hollow portion rather than being straight. As can be clearly seen in FIGURE 4, expansion of the blank by this method permits the unjoined portions of the blank which are relatively wide to be outwardly distended to a greater height than the relatively narrow portions. Such distention is obtained without the use of special dies conforming to the ultimate desired configuration of the hollow portion of the article while preventing the blank from warping or pulling in during inflation thereof.

Referring to FIGURE 6, should it be desired to inflate a very wide unjoined area 43 in conjunction with relatively narrow unjoined areas 45, the compressible pads 47 may be provided with more highly compressible inserts 49. The primary purpose in providing inserts 49 is to permit suflicient outward distention of the relatively wide unjoined portion of the blank 51. For example, if the blank shown in FIGURE 6 was expanded between the pressure pads 35 shown in FIGURE 4, the relatively wide portion of the blank would not expand to its fullpossible height. Thus, the full height of the relatively wide portion is realized and this portion is still restrained against rupturing when exposed to the pressure required to distend the narrow tubes of the blank.

In this embodiment the backing plates 51 have side walls 53 which engage the side walls 55 of the pressure pads 47. Shims may be used between the horizontal faces of the side walls of the backing plates to adjust the compression exerted by the pressure pads on the blank. The side walls 53 of the backing plate 51 restrain the pressure pads from horizontal deformation when the pads are clamped against a blank. In the modification shown in FIGURE 4, the pressure pads 35 are free to expand horizontally and the blank 33 must be spaced inwardly from the side walls of the pressure pads a greater amount than is required in the modification shown in FIGURE 6.

FIGURE 7 shows another embodiment of a finished hollow article 57. In this modification, opposite ends of tapered tubes 59 open, respectively, into a relatively large header 61 and a relatively small header 63. A nozzle receiving strip 65 connects an edge 67 of the article with the end of the smaller header 63. FIGURE 8 shows, in section, the blank 59 from which the hollow article 57 is formed, positioned between a pair of compressible pressure pads 71. Each pressure pad has a cut-out taper throughout the larger portion of its width to conform generally to the taper of the expanded tubes 59. The horizontal portion of the pads is connected with the tapered portion by a second tapered portion 75.

The blank 79 is so positioned with respect to the second tapered portions 75 of the pads 71 that upon expansion of the unjoined portion of the blank the larger header 61 will be externally restrained against rupturing by the tapered portion 75, as shown in.-FIGURE 9. FIGURES 6 and 9 will more clearly indicate the function of the pads in preventing rupturing of the panel during expansion of the unjoined portion thereof. In FIGURE 9, the side 77 of the large header 61 disthe unjoined area.

FIGURES 10 through 14 illustrate the inflation of an arched blank 81, rather than a flat blank. The arching may be accomplished in any known manner. For example, a blank may be arched as suggested in the patent to Leavenworth, U.S. 562,309, granted June 16, 1896, or iri the patent to Rehbein, US. 1,267,591, granted May 28, 1918. FIGURES 11 and 12. show an embodiment of a portion of a clamping press for use in expansion of the arched blank 81. As in the previous embodiments, the pressure pads 83 between which the arched blank 81 is clamped are of compressible material and the configuration of the clamping portion of these pads conforms generally to the arched configuration of the blank shown in FIGURE 10. The firm backing plates by which the compressible pressure pads 83 are backed up conform generally to the arched configuration of the blank 81 because as shown the compressible pressure pads are relatively thin and of substantially consistent thickness. Alternatively, the rigid backing plates may be fiat or of some other con-figuration not conforming to the shape of the blank and the pads will then have blank engaging faces conforming to the blank and a backing plate configuration conforming to the shape of the backing plate. FIGURE 11 is a sectional view with the blank 81 positioned on the lower pressure pad 83 prior to clamping the blank between the pads. FIGURE 12 shows the pads 83 clamped against the blank 81 after the interior portion of the blank has been expanded as previously described.

FIGURES 13 and 14 show another embodiment of a pair of compressible pressure pads 87. In this embodiment, the pads are relatively much thicker than in any of the other embodiments and are. again backed up by rigid backing plates 89. In FIGURE 14, the pads have been brought together clamping the blank 81 therebetween and the unjoined interior blank has been expanded.

For fully annealed soft copper or aluminum alloy blanks, about .060 inch thick, rubber pads having a durometer hardness between 20 and 70 have been found to be adequate. Depending on the particular configura tion of the hollow portion of the article, pull-in of the blank toward the distended portions during expansion of the unjoined portion may not be objectionable in which event the engaging faces of the blank and pads may be provided with a lubricant such as water, graphite powder, colloidal and semicolloidal graphite water suspensions or talc in the event that rubber pads are used. Even though a lubricant is used, the pads will prevent warping of the blank.

If the pressure pads are provided with a cut-out portion such as that shown in the tapered portion of FIG- URE 8, the unjoined portion of the blank adjacent thereto will expand with less resistance because of the cutout. Such a cut-out may be desirable if it is not convenient to use very thick pressure pads. For example, as the accumulator in FIGURE 7 expands, the compression limit of the pad would be reached if the cut-out were not provided and this would result in tending to flatten out and limit the outward expansion of the accumulator portion of the hollow article. Of course, if such flattening is desired, the blank may be expanded without the benefit of such a cut-out portion or with a cut-out portion in just one pad thereby permitting greater outward distention of the blank on the side having the cut-out portion.

As is most clearly indicated in FIGURES 13 and 14, by providing pressure pads of greater thickness it would only be necessary to cut out portions of the pads or resort to specially shaped pads for extremely arched blanks or blanks having extremely wide unjoined portions. In

all of the various embodiments of blanks and pressure pads, the backing plates as shown in FIGURES 4 and 6 are interchangeable. tionbeing that the pressure pads used in conjunction with The only important considera backing plates not having restraining side walls must i one skilled in the'art and the invention is, therefore, not

to be limited bysuch embodiments, materials or details except as set forth in the appended claims. a

I claim:

v1. A method of fabricating a hollow panel from a blank formed of a plurality of sheets having an unjoined interior portion defined and contained within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted 'to contain a fluid under. pressure; said method comprising clamping said blank between opposed compressible pressure pads wherein each of said pads is opposite and coextensive with said blank at least in the area of the outer faces of said blank opposite said unjoined portion, limiting the degree of compressibility of each of said pads to a degree that under an expanding fluid pressure injected into said unjoined portion the holding pressure exerted by each of said pads on said blank is suflicient to prevent rupture of said blank'but sufficiently low that both said pads are impressible in conformity to the distention of said blank obtained upon expansion of said unjoined portion by said'fluid pressure, injecting into said unjoined portion through said inlet said fluid pressure to expand said unjoined portion, and concurrently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while-maintaining sufficient difference between said holding pressure and said fluid pressure, to, both, prevent collapse of said expanded un-' joined portion by the pressure of said pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

2. The method of claim 1 wherein said blank is curved and said pressure pads have a corresponding cooperat ing curvature.

3. The method of claim 1 wherein said blank is curved and said pressure pads have a configuration other than that of the surfaces of the blank which engages the pads.

4. The method of claim 1 wherein said blank is substantially flat and said compressible pressure pads are platens. r

. A method of fabricating a hollow panel from a blank formed of a plurality of sheets having an unjoined interior portion defined and contained within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted to contain a fluid under pressure; said method comprising clamping said blank between opposed compressible pressure pads wherein each of said pads is opposite and coextensive with said blank at least inthe area of the outer faces of said blank opposite said unjoined portion, initially compressing said pads against said blank to limit the degree of compressibility of said pads so that under an expanding fluid pressure injected into. said unjoined portion the holding pressure exerted by each of said pads on said blank is sufiicient to prevent 7 said fluid pressure, injecting into said unjoined portion through said inlet said fluid pressure to expand said unjoined portion, and concun'ently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining suhicient difference between said holding pressure and said fluid pressure to, both, prevent collapse of said expand-ed unjoined portion by the pressure of said pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

6. The method of claim 5 wherein said pads are resilient. a

7. The method of claim 6 wherein pads are rubber-like material.

8. The method of claim 7 wherein said blank is about .060 inch thick fully annealed 28 aluminum and said rubber has a durometer hardness of between 20 and 70.

9. A method for fabricating a hollow panel from a blank formed of a plurality of sheets having an unjoined interior portion defined and contained within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted to contain a fluid under pressure, said unjoined portion comprising a relatively wide and a relatively narrow interconnected area; said method comprising clamping said blank between opposed compressible pressure pads wherein each of said pads is opposite and coextensive with the area of the outer faces of said blank opposite said unjoined portion, limiting the compressibility of each of said pads to a degree that under an expanding fluid pressure injected into said unjoined portion the holding pressure exerted by each of said pads on said blank is suiflcient to prevent rupture of said blank but sufliciently low that both said pads are impressible in conformity to the distention of said blank upon expansionof said unjoined portion by said fluid pressure, injecting into said unjoined portion'through said inlet said fluid pressure to expand said unjoined portion whereby the distention of said relatively wide unjoined portion is impressed into said pads to a greater depth than the depth holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining a sufiicient difference between said holding pressure and said fluid pressure to, both, prevent collapse of said expanded unjoined portion by the pressure of said pads and to preventrupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

10. A method of fabricating a hollow panel from a blank formed of a plurality of sheets having an. unjoined interior portion defined and 'contained'within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted to contain a fluid under pressure, said method comprising clamping said blank between opposed compressible pressure pads wherein each of said pads is opposite and coextensive with said blank at least in the area of the outer faces of said blank opposite said unjoined portion, said pads being recessed in the central portion thereof in conformity to the non-uniformity desired in the height of the area of said unjoined portion opposite the recess in said pads whereby the peripheral edges of said pads adjacent said recess olampsjsaid blank about the peripheral area of said unjoined portion opposite said edges, limiting the degree of compressibility of each of said pads so that upon expansion of sad unjoined portion by injection of an expanding fluid pressure thereinto the holding pressures exerted by each of said pads on the area of said blank opposite said unjoined portion is suflicient to prevent ruptureof said blank but sufficiently lowthat both pads are impressible in conformity to the distention of said blank upon expansion of said unjoined portion by said fluid pressure, injecting into said unjoined portion through said inlet said fluid pressure to expand said unjoined portion into said pads whereby said area of said unjoined portion opposite said recess is expanded subject to less external retardation than sm'd peripheral areas of said blank, and concurrently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining sufiicient difference between said holding pressure and said fluid pressure to, both, prevent collapse of said expanded unjoined portion by the pressure of said pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

11. The method of fabricating a hollow article by distention of a blank having an unjoined interior portion comprising applying to the surface of a sheet of metal a pattern of weld inhibiting material defining the desired configuration of said unjoined interior portion adapted to contain a fluid under pressure, superimposing a second sheet of metal on the surface of said first sheet to which said weld inhibiting material has been applied, welding said sheets together in the areas not separated by said weld inhibiting material to form an integral lank containing said unjoined interior portion, clamping said blank between opposite compressible pressure pads wherein each of said pads is opposite and coextensive with said blank at lea t in the area of the outer faces of said blank opposite said unjoined portion, limiting the compressibility of each of said pads to a degree that under an expanding fluid pressure injected into said unjoined portion the holding pressure exerted by each of said pads on said blank is su fflcient to prevent rupture of said blank but sufficiently low that both said pads are irnpressible in conformity to the distention of said blank obtained upon expansion of said unjoined portion by said fluid pressure, injecting into said unjoined portion through said inlet said pressure to expand said unjoined portion, and concurrently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining sufficient diflerence between said holding pressure and said fluid pressure to, both, prevent coilapse of said expanded unjoined portion by the pressure of said pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

12. A method of fabricating a hollow panel from a blank formed of a plurality of sheets having an unjoined interior portion defined and contained within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted to contain a fluid under pressure; said method comprising non-uniformly retarding expansion of said unjoined portion by clamping the opposed edges of said blank adjacent said unjoined portion between opposed compressible pressure pads wherein each of said pads is opposite and coextensive with the area of the outer faces of said blank opposite said unjoined portion with said pads having their surfaces opposite said unjoined area receding from the faces of said blank in conformance to non-uniformity desired in the expanded height of said unjoined portion, limiting the compressibility of each of said pads to a degree that under an expanding fluid pressure injected into said unjoined portion the holding pressure exerted by each of said pads is sufliicient to prevent rupture of said blank but sufliciently low that both said pads are impressible in conformity to the distention of said blank obtained upon expansion of said unjoined portion by said fluid pressure, injecting into said unjoined portion through said inlet said fluid pressure to outwardly expand said unjoined portion into said pads whereby said unjoined portion expands to a predetermined non-uniformity in height, and concurrently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining suflicient difference between said holding pressure and said fluid pressure to, both, prevent collapse of said expanded unjoined portion by the pressure of said pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

13. A method of fabricating a hollow panel from a blank formed of a plurality of sheets having an unjoined interior portion defined and contained within joined portions of said sheets provided with an inlet for fluid pressure to said unjoined portion with said unjoined portion being adapted to contain a fluid under pressure; said method comprising clamping said blank between op posed compressible pressure pads wherein each of said compressible pads is opposite and coextensive with the area of the outer faces of said blank opposite said unjoined portion and said compressible pads being constructed from a plurality of individual pads of different compressibilities which are positioned adjacent the surfaces of said blank at points of non-uniformity desired in the height of the expanded unjoined portion of said blank, limiting the compressibility of each of said pads to a degree that under an expanding fluid pressure injected into said unjoined portion the holding pressure exerted by each of said compressible pads on said blank is sufficient to prevent rupture of said blank but sufficiently low that both said compressible pads are impressible in conformity to the distention of said blank obtained upon expansion of said unjoined portion by said fluid pressure, injecting into said unjoined portion through said inlet said fluid pressure to expand said unjoined portion into said compressible pads whereby said unjoined portion expands to a height dependent on the compressibility of said individual pads, and concurrently releasing said holding pressure applied on said blank by said pads and said fluid pressure applied within said unjoined portion while maintaining sufficient dilference between said holding pressure and said fluid pressure to, both, prevent collapse of said expanded unjoined portion by the pressure of said compressible pads and to prevent rupturing said expanded unjoined portion by the fluid pressure within said unjoined portion.

References cues in the file of this patent UNITED STATES PATENTS 1,625,914 Seibt Apr. 26, 1927 2,317,869 Walton Apr. 27, 1943 2,416,015 McGuffey Feb. 18, 1947 2,479,702 Rood Aug. 23, 1949 2,582,358 Schoellerman Jan. 15, 1952 2,662,273 Long Dec. 15, 1953 2,690,002 Grenell Sept. 28, 1954 2,728,317 Clevenger et al Dec. 27, 1955 2,740,188 Simmons Apr. 3, 1956 2,741,205 Paulton Apr. 10, 1956 2,749,867 Engel June 12, 1956 2,783,727 Hoffman Mar. 5, 1957 2,857,660 Staples Oct. 28, 1958 

